The present invention relates to arrangements for freeing or protecting a semiconductor switch from high forward blocking voltage stresses, often referred to as voltage spikes or voltage transients, which occur when the semiconductor switch changes from a conducting state to a non-conducting or blocking state. More particularly, the present invention relates to a system which protects the semiconductor switch against such voltage stresses or spikes and which provides for utilization of the energy which creates the voltage spikes, thus increasing the efficiency of the system.
In the past, circuits have been provided for freeing or protecting a semiconductor switch from high forward blocking voltage stresses, i.e., voltage spikes or transients, which occur as the switch is turned off. For example, DE-OS No. 3,609,375 discloses an arrangement for freeing or protecting a semiconductor switch from such high forward blocking voltages by providing a capacitor-diode circuit, called a "snubber", to absorb the energy during turn-off of the semiconductor switch by charging the capacitor of the snubber. The diode of this snubber is connected to a current converter transformer, and the capacitor of the snubber is discharged through the current converter transformer and is fed back to a voltage source, e.g. the voltage source driving the load current, by way of a useful impedance.
in arrangements according to DE-OS No. 2,644,715, oscillator circuits are provided for protecting the semiconductor switches. The oscillator circuits are composed of capacitors, inductances and diodes, and although the arrangements according to DE-OS No. 2,644,715 permit utilization of the energy during turn-off of the semiconductor switch, the arrangements require expensive circuitry. Furthmore, the parameters for such circuits are often critical.
U.S. Pat. No. 4,700,083 discloses a semiconductor switch which is equipped with a voltage limiting device including an oscillator circuit. It is also known to protect a semiconductor switch against excess voltage stresses by equipping it with a so-called RCD network (DE-OS No. 3,538,184; Electronic Design News, May 1981, pages 126-130). When using an RCD network the protection is realized by the dissipation of energy in an ohmic resistor.